Questions answered

Sherry Seethaler, Special to the Union-Tribune

QUESTION: Why do nuclear reactors and nuclear-fission bombs use uranium or plutonium rather than thorium? The fact that almost all natural thorium is 232 would be a huge advantage, because there would be no need to enrich uranium or manufacture plutonium.

— John Myers

ANSWER: Had nuclear-power technology not grown up during the Cold War, thorium reactors might now be mainstream. Back then, nuclear reactors fueled by uranium made more sense because they could be used to make plutonium, which at the time was in demand for making bombs.

Uranium fuel must be enriched in uranium-235 because less than 1 percent of natural uranium is fissile uranium-235, and the more abundant uranium-238 does not spontaneously split, or fission. Yet thorium does not overcome this challenge. Like uranium-238, thorium-232 is not itself fissile. This makes it unsuitable for fission bombs. Also, it must be exposed to neutrons to convert it into a nuclear fuel.

To provide the initial neutrons to get things started in a thorium reactor, the only practical solution at the moment is to include conventional nuclear fuel, plutonium and/or enriched uranium. In a conventional reactor, neutron capture converts uranium-238 into plutonium. In a thorium reactor, thorium-232 is converted into uranium-233, which is fissile.

Thorium-based reactors have at least three potential advantages. First thorium is three or four times more abundant than uranium in the Earth’s crust, and present in deposits that are easier to mine than those containing uranium.

Second, although uranium reactors and thorium reactors both accumulate many radioactive waste products, the waste products of a thorium reactor decay more quickly and are produced in lower amounts.

Third, should thorium-reactor waste fall into the wrong hands, it is more difficult (though not impossible as some sources have claimed) to produce a bomb. The technology needed to separate plutonium from traditional reactor waste is more accessible than the technology required to separate uranium-233 from thorium reactor waste. This is because uranium-232, which is generated in small quantities along with uranium-233, cannot be removed by simple means and has gamma-ray-emitting decay products that would damage the electronics of a bomb.

At present, the economics of thorium reactors are not clear cut. Many different types of thorium reactors have been tested, from completely novel designs to fuel rod assemblies that can be used in current water-cooled reactors with minimal modification. In the long-term, completely new reactor designs may be more efficient than using thorium in current reactor types, but a large investment of time and money is required to license and introduce a new type of commercial reactor.